1. Field of the Invention
Sulfur has always been an objectionable constituent of coal and other carbonaceous materials normally used to generate power and energy. With the rapidly diminishing oil supply and the present world-wide emphasis on the energy crisis, increased attention by both government and private organizations is being given to coal as an energy source. However, combustion of coal for heat, steam, and electrical power generation is by far the largest single source of atmospheric sulfur dioxide pollution in the United States, accounting for approximately two-thirds of the total sulfur oxide emissions.
Depending upon their origin, coals contain varying amounts of sulfur in the form of pyrite (iron disulfide), organic sulfur and sulfate sulfur from which sulfur dioxide is formed when coal is burned. Recent environmental considerations have led to legislation requiring the removal or substantial reduction of sulfur levels in fuels, such as coal, prior to their combustion. One of the principal drawbacks in the use of coals as a fuel source in the United States is due to their sulfur content which can range up to as high as ten percent. Hence, there is an ongoing search to convert coal directly into a usable fuel which would meet current air quality standards. The best processes which are known to the art, however, generally remove small levels of the total amount of sulfur in coal, especially organic sulfur compounds.
2. Brief Description of the Prior Art
The problem of separating sulfur and/or other impurities from coal and carbonaceous materials is not new and several processes have been extensively tested over the years. One such process is set forth in U.S. Pat. No. 4,083,696 which discloses a process for reducing the pyritic sulfur content of an aqueous coal slurry under conditions of turbulent flow through a pipeline. This is accomplished by introducing a pyrite oxidant into the pipeline upstream from a coal dewatering plant. Sutiable pyrite oxidants include hydrogen peroxide, ferric chloride, nitric acid, etc. The process is conducted at ambient pressure and at a temperature of from about 70.degree. F. to about 100.degree. F.
Another approach for separating sulfur from coal and carbonaceous materials is set forth in U.S. Pat. No. 3,909,213 which relates to a process for desulfurization of coal wherein coal and a Group IA or IIA metal oxide and a fused metal chloride salt are digested in a digestion zone. Representative examples of metal sulfides and metal chlorides include calcium or sodium oxide and zinc or ferric chloride.
U.S. Pat. No. 3,926,575 discloses a process for reducing a pyritic content of coal which comprises reacting finely divided coal with sulfuric or hydrochloric acid. An alternative method is disclosed for removing sulfur from coal which involves, in addition to an acid leach, a final extraction step involving use of a solvent, for example, benzene, etc.
U.S. Pat. No. 4,071,328 relates to a method of removing sulfur from coal which comprises hydrogenating the coal to remove a portion of sulfur therefrom. The hydrogenated coal is next contacted with an aqueous inorganic acid solution to remove some of the sulfur initially combined as pyritic sulfur.
It is readily apparent from the above that there is an ongoing search for newer, more efficient and more economical methods for removing sulfur from coal and carbonaceous materials.